by Jolie Lee, USA TODAY Network

by Jolie Lee, USA TODAY Network

The Ebola virus infecting and killing people in West Africa is the worst recorded outbreak, according to the World Health Organization. The first case of Ebola in a patient diagnosed in a U.S. hospital was confirmed by the Centers for Disease Control and Prevention Tuesday.

USA TODAY Network explains why the virus is so deadly and how likely it is to spread in the USA.

What are the symptoms?

Initial symptoms include fever, intense weakness, muscle pain, headache and sore throat. These symptoms are followed by vomiting, diarrhea, rash, impaired kidney and liver function and sometimes internal and external bleeding, according to WHO.

Symptoms usually appear 8-10 days after infection, according to the CDC. WHO says lab tests of contaminated individuals find low white blood cell and platelet counts.

How does the virus spread?

The virus is transmitted from wild animals to humans. Humans spread the virus through contact with blood or other body fluids of an infected person, as well as exposure to objects like contaminated needles, according to the CDC.

Ebola is only contagious when patients begin showing symptoms. Infection control measures in place in U.S. hospitals means it's unlikely the virus will spread far, according to infectious disease experts. People most at risk include health workers and family members or others who are in contact with the infected people, according to WHO.

What is the treatment?

Currently, there is no vaccine. Treatment consists only of "supportive therapy," according to the CDC. There are several vaccines being tested. None are available for clinical use.

How deadly is Ebola?

In past outbreaks, up to 90% of humans who contract the virus have died. WHO describes Ebola as "one of the world's most virulent diseases."

In the current outbreak, the survival rate has been higher. About 47% survive, according to WHO.

How can Ebola be prevented?

It's unknown what the natural host for Ebola is, but it's believed to be the fruit bat. If an outbreak among animals is suspected, the best practice is to quarantine the animals, cull the infected animals and bury or incinerate the carcasses, according to WHO.

In the current outbreak, most cases are the result of human-to-human transmission, when there is direct contact with bodily fluids, secretions, the mucous membrane or broken skin of an infected person, WHO says.